A major complication of cancer chemotherapy and of antiviral chemotherapy is damage to bone marrow cells or suppression of their function. Specifically, chemotherapy damages or destroys hematopoietic precursor cells, primarily found in the bone marrow and spleen, impairing the production of new blood cells (granulocytes, lymphocytes, erythrocytes, monocytes, platelets, etc.). Treatment of cancer patients with 5-fluorouracil, for example, reduces the number of leukocytes (lymphocytes and/or granulocytes), and can result in enhanced susceptibility of the patients to infection. Many cancer patients die of infection or other consequences of hematopoietic failure subsequent to chemotherapy. Chemotherapeutic agents can also result in subnormal formation of platelets which produces a propensity toward hemorrhage. Inhibition of erythrocyte production can result in anemia. The risk of damage to the hematopoietic system or other important tissues can prevent utilization of doses of chemotherapy agents high enough to provide good antitumor or antiviral efficacy.
Many antineoplastic or antiviral chemotherapy agents act by inhibiting nucleotide biosynthesis, metabolism, or function, or are in fact nucleoside analogs that substitute for the normal nucleosides in nucleic acids, producing defective RNA or DNA.
5-Fluorouracil is a clinically important cytoreductive antineoplastic chemotherapy agent that acts in part through incorporation into RNA, producing defective RNA; inhibition of thymidylate synthetase by fluorodeoxyuridine monophosphate may also contribute to the cytotoxicity of 5-FU. The clinical utility of 5-FU is limited by its toxicity (especially to bone marrow). Specifically, its clinical utility is limited by a low therapeutic ratio (the ratio of toxic dose to effective dose; a high therapeutic ratio implies that a drug has efficacy with little toxicity).
5-FU and many other chemotherapy agents also affect other tissues, especially gastrointestinal mucosa, producing mucositis, diarrhea and ulceration. Stomatitis (ulceration of mucosa in the mouth), is particularly troublesome to patients, making eating and swallowing painful.
D. S. Martin et al. (Cancer Res. 42: 3964-70 [1982]) reported that a toxic dose of 5-FU (with strong anti-tumor activity) could be safely administered to mice if followed by administration of a high dose of uridine beginning several hours later. This "rescue" strategy has been shown to increase the therapeutic index of 5-FU in animal tumor models, allowing administration of the high, toxic doses of 5-FU that are necessary for causing tumor regresssion or preventing tumor growth while preferentially protecting normal tissues (especially important is bone marrow) by subsequent administration of uridine (D. S. Martin et al., Cancer Res. 43: 4653-61 [1983]).
Clinical trials involving the administration of uridine have been complicated due to the biological properties of uridine itself. Uridine is poorly absorbed after oral administration; diarrhea is dose limiting in humans (van Groeningen et al., Proceedings of the AACR 28: 195 [1987]). Consequently, parenteral administration of uridine is necessary for clinically significant reversal of 5-FU toxicity, which requires use of a central venous catheter, since phlebitis has been a problem in early clinical trials when uridine was administered via a small intravenous catheter (van Groeningen et al. Cancer Treat Rep. 70: 745-50 [1986]). Prolonged infusion via central venous catheters requires hospitalization of the patients. Further, there is considerable discomfort and inconvenience to the patients.
Orally-active prodrugs of 5FU have been developed which are enzymatically or spontaneously converted to 5FU, generally after absorption from the intestine into the bloodstream. This permits self-administration by patients, without the discomfort of intravenous administration. Moreover, in some chemotherapy regimens, sustained exposure, e.g. a constant intravenous infusion for several days or weeks, of tumors to 5FU is attempted. Oral administration of 5FU prodrugs can in principle provide such sustained availability of 5FU to tumors.
5-Fluoro-1-(tetrahydro-2-furfuryl)uracil (FT) is an orally active prodrug of 5-fluorouracil. It is enzymatically converted to 5-fluorouracil primarily in the liver. The liver, however, has relatively high levels of the enzyme dihydropyrimidine dehydrogenase, which degrades 5FU, producing metabolites which are not useful in cancer chemotherapy and which furthermore contribute to 5-FU toxicity.
The cytotoxicity of 5FU, the active metabolite of FT, is believed to be a result of its incorporation into nucleotide pools, where certain anabolites exert toxic effects, e.g. 5-fluorodeoxyuridine monophosphate inhibits thymidylate synthetase, thus depriving cells of thymidine for DNA synthesis, and 5-fluorouridine triphosphate incorporation into RNA impairs its normal functions in translation of genetic information.
In order to inhibit the catabolism of 5FU derived from FT, other compounds have been administered with the FT. In particular, the pyrimidine uracil inhibits the catabolism of 5FU without inhibiting its cytotoxicity. The most widely used clinical formulation of FT contains uracil in a 1:4 molar ratio. This permits a significant reduction in the dose of FT required to achieve a therapeutic effect. Other pyridimines, including uridine, thymidine, thymine, and cytosine are either less effective than uracil or no better in potentiating the antitumor efficacy of FT without unacceptably potentiating toxicity. Potent synthetic inhibitors of dihydropyrimidine dehydrogenase (DHPDHase) have also been utilized with FT or 5FU. 5-chloro-2,6-dihydroxypyridine (CDHP) is more potent than uracil as an inhibitor of DHPDHase. However, this compound also enhances the toxicity of 5FU, so that, in its intended clinical implementation, a third component, oxonic acid, is co-administered to reduce the intestinal toxicity.
Several investigators have administered pyrimidines with 5FU attempting to improve the therapeutic index of this antineoplastic agent. In vivo, uridine and thymidine when administered at the same time as 5FU increased both the antitumor efficacy of 5FU and its toxicity, so that there was no net increase in therapeutic index (Hartman and Bollag, Med. Oncol. & Tumor Pharmacother., 3: 111-118 [1986]). Burchenal et al. (Cancer Chemother. Rep., 6: 1-5 [1960]) summarized comprehensive studies on interactions of 5FU and 5-fluorodeoxyuridine (FUDR) and pyrimidine compounds. They noted that despite the fact that pyrimidines and pyrimidine nucleosides, at doses which are inactive alone, markedly potentiate the antileukemic effects of small doses of FUDR or FU, it has not been possible with any combination to improve significantly and with any degree of regularity the results which can be obtained with maximum tolerated doses of FU or FUDR alone. Similarly, Jato et al. (J. Pharm Sci., 64: 943-945 [1975]), in an investigation of combinations of deoxyuridine with 5FU and FUDR report that any therapeutic benefit of the combination therapy could be duplicated with either 5FU or FUDR at a higher dose. Although deoxyuridine, by inhibiting the catabolism of the fluoropyrimdines permitted adminstration of lower doses, deoxyuridine there was no improvement in antitumor activity at equitoxic doses of the combination versus FU or FUDR alone.
As in the case of uridine, problems of poor bioavailability after oral administration limit the clinical utility of administration of deoxycytidine, cytidine, and deoxyuridine themselves for modulation of toxicity of chemotherapy agents.
Arabinosyl cytosine (Ara-C) is an important agent in the treatment of leukemia, and is also useful as an immunosuppressant. Bone marrow toxicity (myeloid and erythroid) associated with Ara-C administration can be partially prevented by administration of deoxycytidine (Belyanchikova et al. Bull. Exp. Biol. Med. 91: 83-85 [1981]), while the toxicity of Ara-C to lymphocytes is not as strongly attenuated by deoxycytidine. In cell cultures, normal myeloid progenitor cells are protected from Ara-C by deoxycytidine better than are leukemic cells (K. Bhalla et al. Blood 70: 568-571 [1987]). Deoxycytidine also attenuates toxicity of 5-aza-2'-deoxycytidine and arabinosyl 5-azacytosine in cell cultures (K. Bhalla et al. Leukemia 1: 814-819 [1987]). Prolonged (5 day) infusion of high doses of deoxycytidine via a central venous catheter was proposed as a means for clinical implementation of modulation of Ara-C toxicity with deoxycytidine (K. Bhalla et al. Leukemia 2: 709-710 [1988]).
N-phosphonoacetyl-L-aspartic acid (PALA) is an antineoplastic agent that inhibits the enzyme aspartate transcarbamoylase, an enzyme indirectly involved in biosynthesis of pyrimidine nucleotides. Side effects of PALA primarily involve damage to gastrointestinal toxicity and mucositis. Pyrazofurin (a carbon linked pyrimidine analog), 6-azauridine, and 6-azacytidine all interfere with pyrimidine nucleotide synthesis and metabolism.
3'-Azidodeoxythymidine (AZT) is used clinically in patients infected with Human Immunodeficiency Virus (HIV, the infectious agent in AIDS). AZT prolongs the lifespan of patients infected with HIV, but also impairs hematopoiesis, producing leukopenia and anemia. In cell cultures, uridine ameliorates AZT-induced toxicity to granulocyte/macrophage progenitor cells without impairing the antiviral actions of AZT (Sommadossi et al., (1988) Antimicrobial Agents and Chemotherapy, 32: 997-1001); thymidine attenuated both toxicity and antiviral activity. In mice, parenteral administration of high doses of uridine provided some amelioration of AZT-induced anemia, but only at uridine doses which increased mortality during the study; a low, non-toxic dose of uridine (500 mg/kg/d) did not reduce AZT-induced hematologic toxicity (A. Falcone, et al. Blood 76: 2216-21 [1990]). Sommadossi and el Kouni (U.S. Pat. No. 5,077,280) proposed the administration of uridine by periodic intravenous injection in order to attenuate AZT toxicity. Bhalla et al. (Blood 74: 1923-1928 [1989]) reported that deoxycytidine protects normal human bone marrow progenitor cells in vitro against the cytotoxicity of AZT with preservation of antiretroviral activity.
5-Fluoroorotate, an analog of the pyrimidine nucleotide precursor orotic acid, has antiproliferative effects on human cells, but is especially useful for treating infections with malarial parasites, e.g., Plasmodium yoelii or Plasmodium falciparum, which are dependent on de novo pyrimidine biosynthesis. Administration of uridine to mice treated with 5-fluoroorotate attenuated host toxicity due to the latter without impairing its antimalarial activity (ZM Gomez and PK Rathod, Antimicrob. Agents Chemother. 34: 1371-1375 (1990).
Dideoxycytidine (ddC) is also useful against retroviral infections including HIV; side effects of ddC include peripheral neuropathy, mouth ulcers, and reduced platelet counts. The toxicity of ddC on human myeloid progenitor cells in culture can be ameliorated by deoxycytidine without thereby impairing the antiretroviral efficacy of ddC (K. Bhalla et al., AIDS 4: 427-31 [1990]).
The methods disclosed in the prior art cited above for administering these pyrimidine nucleosides to modify chemotherapy in the clinical setting are neither practical (prolonged infusion of deoxycytidine or uridine via a central venous catheter requires hospitalization, risk of infection, and discomfort to the patient) or satisfactory (orally administered uridine is poorly absorbed; therapeutically adequate doses of oral uridine produce diarrhea).
Commonly owned U.S. patent application Ser. No. 438,493 demonstrates the use of acylated derivatives of cytidine and uridine to increase blood cytidine or uridine levels.
Some acyl derivatives of pyrimidine nucleosides have been synthesized for use as protected intermediates in the synthesis of oligonucleotides or nucleoside analogs, e.g. 5'-O-benzoyluridine, triacetylcytidine, and triacetyluridine. See Sigma Chemical Company 1991 catalog, pages 155, 980, and 981 respectively.